The invention relates generally to the measurement of coating thickness employing eddy current techniques and, more particularly, to an eddy current probe for measuring the thickness of conductive coatings on conductive substrates, as well as to a measurement system and method using the same.
Components, such as airfoils used for gas and steam turbine power generator applications, include protective coatings that degrade with use. The condition of the coating is critical to the utility of the airfoil. Accordingly, both manufacturers and users must be able to assess the protective coating thickness on the airfoil. Ideally, the coating thickness should be measurable at any point in the service life of the airfoil. In-situ measurements are also important to minimize inspection costs.
For many such components, e.g., the airfoils, both the component and the protective coating are formed of conductive materials. Furthermore, the materials used to form the conductive coating and component often have similar properties.
One example is an airfoil formed of GTD111, a nickel based super alloy, and coated with GT29, a nickel based alloy. Because of their similar properties, it is difficult to distinguish the conductive coating from the conductive component without resort to destructive measurement techniques.
Consequently, the thickness of the conductive, protective coating on conductive airfoils is currently measured by performing metallographic sectioning, which destroys the airfoil. A second method involves weighing the airfoil before and after application of the conductive coating and then using successive weight loss measurements to estimate coating erosion. Although the second method is nondestructive, it is deficient, in that weight loss merely indicates overall coating loss and cannot isolate critical areas, such as the leading and trailing edges of the airfoil. In addition weight measurements also require that the airfoils be removed from the rotor.
Accordingly, it would be desirable to employ a non-destructive measurement technique to determine the thickness of a conductive coating on a conductive component, such as an airfoil. Further, it would be advantageous to provide a measurement technique that can be used to isolate critical areas on the component and that does not require disassembly of the component from its surroundings.
Presently, eddy current inspection provides a non-destructive technique for performing a different class of evaluations, namely the detection of discontinuities or flaws in the surface of various components. See, for example, Hedengren, et al., U.S. Pat. No. 5,389,876, entitled xe2x80x9cFlexible Eddy Current Surface Measurement Array for Detecting Near Surface Flaws in a Conductive Partxe2x80x9d and Hurley et al., U.S. Pat. No. 5,510709, entitled xe2x80x9cEddy Current Surface Inspection Probe for Aircraft Fastener Inspection and Inspection Method.xe2x80x9d Briefly, eddy current inspection is based on the principle of electromagnetic induction in which a drive coil is employed to induce eddy currents within the material under inspection, and secondary magnetic fields resulting from the eddy currents are detected by a sense coil, generating signals which are subsequently processed. The drive and sense coil are distinct eddy current coils for differential measurements. For example, oppositely wound drive and sense coils can be used to produce a differential signal. In contrast, the drive and sense coils are provided by the same eddy current coil for absolute measurements.
Eddy current inspection detects flaws as follows. The presence of a discontinuity or a crack in the surface of the component under inspection changes the flow of the eddy currents within the test specimen. The altered eddy current, in turn, produces a modified secondary magnetic field, which is detected by the sense coil, thereby generating a signal which indicates the presence of the flaw upon subsequent processing.
Although this eddy current inspection technique can be extended to measure the thickness of a nonconductive coating on a conductive component, for example, measuring paint thickness on a structure or measuring the thickness of a ceramic coating (thermal barrier coating), this technique is not applicable to conductive coating/conductive substrate combinations. In particular, coating thickness measurements on a nonconductive coating/conductive component combination exploit the fact that the magnetic field generated in the conductive component and detected by an eddy current coil falls off with the thickness of the nonconductive coating. However, this generic methodology is not applicable to conductive coating/conductive substrate combinations because of their similar conductivities.
Accordingly, it would be desirable to develop a measurement technique based on the principle of electromagnetic induction to determine the thickness of a conductive coating on a conductive component, such as an airfoil, in order to exploit the advantages of eddy current inspection. These advantages include: its non-destructive nature and its ability to perform local measurements without removing the component from its environment.
In addition, it would be desirable to develop a sensitive conductive coating thickness measurement technique, which is capable of distinguishing materials having similar electrical properties, such as GTD111 and GT29. It would further be desirable to increase the accuracy of the measurements by providing partial protection from certain environmental factors, such as temperature, pressure, and humidity, which degrade precision.
Briefly, in accordance with one embodiment of the present invention, a self referencing eddy current probe for determining conductive coating thickness includes a housing having a reference sample area, for accommodating a reference sample, and a testing edge, for positioning on a component during a coating thickness measurement. The eddy current probe further includes a reference eddy current coil situated in the housing adjacent to the reference sample area and a test eddy current coil, which is located at the testing edge.
In accordance with a second embodiment of the present invention, a self referencing eddy current measurement system, for measuring a thickness of a conductive coating on a component, includes a self referencing eddy current probe. The probe includes a housing including a reference sample area, for accommodating a sample, and a testing edge, for positioning on a component during a coating thickness measurement. The probe further includes a test eddy current coil located at the testing edge, and a reference eddy current coil situated in the housing adjacent to the reference sample area.
The self referencing eddy current measurement system further includes a signal generator for energizing the test and reference coils. The self referencing eddy current measurement system also includes a comparison module for comparing a test signal received from the test coil and a reference signal received from the reference coil and outputting a compared signal.
In accordance with a third embodiment of the present invention, a self referencing eddy current measurement method includes positioning an edge of an eddy current probe housing on a test object. The method further includes energizing a test eddy current coil facing the test object, the test coil being situated on an edge of the probe housing. The method also includes energizing a reference eddy current coil facing a reference sample, the reference sample and reference coil being situated in the probe housing. The method further includes comparing a test signal from the test coil with a reference signal from the reference coil to produce a compared signal, and converting the compared signal to a coating thickness value.